1. Field of the Invention
The present invention relates to optical disc type discrimination in an optical disc apparatus. More specifically, a device and method for correctly distinguishing optical disc type independent of the velocity of the focusing lens is disclosed.
2. Description of the Prior Art
Many of today's optical disc devices are designed to operate using more than one type of optical disc. For example, a single optical disc drive may be capable of reading/writing at least CD, CD-R, CD-RW, and DVD formats. Because the wavelength of a laser optimized for use with a CD is longer than the wavelength of a laser optimized for use with a DVD, it is of fundamental importance for the modern optical disc apparatus to distinguish automatically, quickly, and correctly, which type of optical disc is currently being utilized. Once the determination has been made, the optical disc apparatus selects the appropriate laser according to the disc type.
Most current optical discs have been standardized to be approximately 12 mm in diameter and about 1.2 mm thick. The optical discs have a protective surface layer and an underlying data layer where the desired information is recorded. Although there may also be differences between the exact structure and composition of the data layer (or layers), one major difference between a CD and a DVD is the respective distance between the surface layer 105, 125 and the data layer 110, 130 as shown in FIG. 1. For a CD 100, this distance is nearly the thickness of the disc, or about 1.2 mm. For a DVD 120, this distance is about half the thickness of the disc, or approximately 0.6 mm.
When determining the type of optical disc currently loaded, the optical disc apparatus normally utilizes a fixed-force to raise and then lower an optical focusing lens within a narrow, predefined range at a constant known velocity. As the lens is moved closer to the optical disc, the focus of the laser moves to the surface layer of the disc, then through the surface layer of the disc, and eventually to the data layer and perhaps beyond. When the direction of motion of the lens is reversed, the focus of the laser again passes through the data layer and eventually through the surface layer of the disc. Because of the different laser wavelengths and relative compositions of CDs and DVDs, a CD will reflect light generated by a laser having a wavelength optimized for a CD better than light from a laser optimized for a DVD. Conversely, a DVD will reflect light from a laser having a wavelength optimized for a DVD better than light generated by a laser optimized for a CD. Reflected light maximizes when the laser focus centers on the data layer or the surface layer.
FIG. 2 illustrates one method to determine optical disc type that uses a focus error (FE) signal. In this example, a CD has been inserted into the optical disc apparatus. Also shown in FIG. 2 is a generated SBAD signal, which is a sum of all reflected light received by sensors in the pickup head of the optical disc apparatus, and an FOSO signal indicating the distance between the lens and the surface of the optical disc. First, a DVD laser is turned on and the lens is moved toward the surface of the disc. As the focus of the laser passes through the surface layer at a point on the chart marked A, reflected light increases somewhat, as is indicated by the SBAD signal. As the laser focus reaches the data layer, marked B, the quantity of reflected light again increases. Note that here, the FE also shows a minor deviation from normal, but the deviation is small. When the movement of the lens toward the disc reaches the end of the predefined range, the DVD laser is turned off, the CD laser is turned on, and lens movement away from the optical disc commences. When the focus of the laser again reaches the data layer (marked C), the levels of reflected light again surges as shown. However, because the optical disc in question happens to be a CD and now a laser with a wavelength optimized for a CD is being used, the FE signal jumps markedly. Because the increase in the FE signal at the data layer is larger with the CD laser than it is with a DVD laser, the optical disc is determined to be a CD.
FIG. 3 illustrates the same scenario when a DVD has been inserted into the optical disc apparatus. The DVD laser is turned on and the lens is forced toward the optical disc. The DVD laser is then turned off and the lens withdrawn from the optical disc. As is clearly shown, this time the largest jump in the FE when the lens is focused on the data layer occurs at B (the FE signal is greater at B than at C). Thus, the unidentified optical disc is determined to be a DVD. Because the method depends entirely upon the relative reflectivity of the different disc layers, consistency in the reflective relationships for each type of optical disc is a requirement often difficult to meet across all optical disc formats, compositions, and manufacturers.
Another common method to distinguish whether a CD or a DVD is being used utilizes the difference in distance between the surface layer and the data layer. U.S. Pat. No. 6,021,102, issued to Seto, et al. and herein incorporated by reference, provides a detailed explanation of one such method. Again, when determining the type of optical disc currently loaded, the optical disc apparatus utilizes a fixed force to raise and then lower an optical lens within a narrow, predefined range at a fixed velocity. However, in this method, using surges in the SBAD signal, the amount of time it takes for the moving lens to shift from being focused on the surface layer until it focuses on the data layer (or visa versa) determines the type of optical disc. Because the velocity of the moving lens is constant and known, once the time is known, the distance between the surface layer and the data layer can be estimated according to the formula Velocity*Time=Distance. The estimated distance indicates the type of disc.
FIG. 4 illustrates an example related time value Tcd between surges in the SBAD signal when a CD has been inserted into the optical disc apparatus. FIG. 5 shows the same information when a DVD is loaded. Because the speed of the lens is constant and the distance between the surface layer A and the data layer B in a CD (FIG. 4) is approximately twice the distance between the surface layer A and the data layer B in a DVD (FIG. 5), the value of TCD is approximately twice the value of TDVD, allowing for identification of the optical disc type.
The first method requires the reflective relationships of the respective discs to be consistent to work well. However, manufacturing tolerances vary widely and this method is not always applicable. The second method, that of using the distance between the surface layer and the data layer to determine optical disc types predominantly removes the reflectivity problem but introduces a new problem of requiring the precise velocity of the lens to be known. The measurements of the distance between the surface layer and the data layer of an optical disc utilizing various discs and apparatus will generally fall within certain ranges depending upon optical disc type. However, again due to variations in manufacturing tolerances and improvements in technologies, the velocity of the lens may vary considerably from manufacturer to manufacturer and model to model. Inconsistency in velocities across manufacturers may force at least portions of ranges indicating optical disc type to overlap. As a result, the optical disc type may be determined incorrectly.